The present invention relates to a foldable optical apparatus, and more particularly to an optical apparatus, such as a portable overhead projector or an image input apparatus, that has an image forming system foldable with respect to a table on which an original is placed.
Overhead projectors or image input apparatus are advantageously used to show viewers simultaneously a picture or notes on an original. Such an overhead projector projects an image of a transparent original onto a remote screen. A recent tendency in such instruments is to provide a head unit containing an image forming unit and a projection unit which is foldable with respect to a table on which an original is placed, for convenience of carrying and storage.
One such overhead projector is described in, for example, Japanese Unexam. Utility Model Publ. No. 61-13,843, having a cover structure with a head unit formed as an integral whole that is folded and unfolded with respect to a table on which an original is placed. When unfolded, the head unit is positioned above the table. The integral head unit is, on the one hand, advantageous since it is brought into a proper position above the table by opening and raising the cover structure but, on the other hand, requires the overhead projector to have a generally box-shaped body housing for receiving the head unit when the cover structure is closed. This makes it difficult to construct the overhead projector so as to be compact when folded.
Some overhead projectors are provided with a swingable head unit attached to a supporting structure which is folded and unfolded with respect to the table. The head unit is, after unfolding the supporting structure, swung so as to locate the projection lens system above the table. In such an overhead projector, it is necessary to incorporate a lock mechanism for locking or latching the supporting structure in the unfolded position before placing the projector lens system above the table. This leads to a troublesome unfolding operation.
To facilitate the unfolding operation, it was proposed to locate the head unit above the table responsive to the unfolding of the supporting structure. In this case, however, it is necessary to provide linkage between the supporting structure and the head unit, resulting in a complicated mechanism of the supporting structure. Such a supporting structure and its associated mechanism neither cover the surface of a box-shaped body housing nor are receivable in the body housing because of the limited available inner space of the body housing, containing as it does various elements or units such as a power supply unit. Accordingly, it is hard to construct such an overhead projector so as to be compact in size.
Since the head unit contains an image forming system such as a projection lens system or an image forming lens system, which is generally heavy in weight, the supporting structure should be assisted by means of a strong spring so as to be unfolded by exerting only a small force and to be refolded in a cushioned manner. The provision of such a spring needs a lock mechanism for keeping the supporting structure folded. The spring, however, gradually loses elasticity with age, and so it becomes difficult to keep the supporting structure unfolded when the lock mechanism is released, allowing the supporting structure to collapse.